This disclosure relates to cement isolation fluids for use in wellbores, methods for their manufacture, and methods of use comprising at least one of the foregoing.
Drilling fluids (or “muds”) used in the drilling of subterranean oil and gas wells and other drilling applications are well known. Drilling fluids carry cuttings and other particulates from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling fluid is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores, and electrical logs. It will be appreciated that as used herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids”.
Plugging oil or gas wells with a cement plug is a common operation in the art. In general, one of the goals of plug cementing is to secure a stable and effective seal in a designated location of the wellbore, generally not at the bottom of the wellbore. In another operation, a cement plug known as a whipstock plug is intended to set up within the well at a desired location. After the cement has hardened, the drilling operation commences with the intent of using the whipstock plug cement as a kickoff point from which to redirect the path of the drill bit and therefore the direction of the wellbore. The cement is accordingly placed in the desired location in the well in the form of a slurry, which then sets to form the cement plug. Placing a relatively small amount of cement slurry above a larger volume of drilling fluid requires consideration of design factors such as the density and rheology of both the cement and the drilling fluid, hole size and hole angle, including vertical, deviated and horizontal well orientations.
Cement spacers and scavenger cement systems are used to aid separating drilling fluid from the cement slurry. Because cement slurries are usually denser than drilling fluids, the lighter drilling fluid tends to migrate upward and through the cement slurry. At the same time, the heavier cement slurry tends to fall in the wellbore as it sets up. This phenomenon is known as density swapping of fluids within the wellbore. Density swapping can ultimately lead to the plug failing to set in its intended place, and the subsequent mixing of wellbore fluids with the plug slurry will have detrimental effects on the intended performance of the set cement. It is common in the case of whipstock plugs that multiple cement plugs are placed within the wellbore before a successful attempt is accomplished. Due to the inefficiency of the plug cementing operation, delays of many hours or days in the drilling and completion of a well can result.
Known cement spacers can have drawbacks such as instability under operating conditions, especially at higher temperatures as they can exist at the bottom of the wellbore. Other prior art cement spacer fluids are not entirely effective, and allow mixing and remixing of the fluids they are designed to separate. Even if the mixing does not result in density swapping, contamination of the cement slurry can slow or prevent setting, such that the set cement may be compromised with respect to its ability to bond to both the exposed rock surface in the drilled wellbore and to the tubulars placed in the wellbore. In another type of operation, a plug may be placed within the casing string and, if compromised with respect to its integrity, it may not remain competent within the well to perform its purpose.
Accordingly, there remains a need in the art for a wellbore fluid that overcomes the aforementioned drawbacks of using cement spacers to isolate cement slurries. It would be a particular advantage if the fluids were stable at high temperatures.